The WCDMA/HSPA and LTE technologies have further evolved and are constantly evolving towards multi-carrier systems. A multi-carrier UE, which has multiple receivers or a wide band receiver, is typically able to measure on inter-frequency carriers without gaps. However such UEs may still require measurement gaps for performing measurements on inter-RAT carriers and also on the additional inter-frequency carriers. The additional inter-frequency carriers are those which cannot be measured without gaps. For example in DC-HSUPA, the UE can measure on secondary DL carrier (i.e. inter-frequency), which is adjacent to the primary DL carrier without gaps. But the same UE requires gaps to measure on inter-frequency which is non adjacent to the DL primary carrier or belong to another band (e.g. band B) if the UE supports band B.
A plurality of carriers of the same radio access technology (RAT) may be aggregated, so called Intra-RAT carrier aggregation, as here discussed. To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MHz carriers in HSPA to enhance the peak-rate within the HSPA network, and work is on-going for LTE Release 10 to facilitate aggregation of multiple LTE carriers. Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier (CC) or sometimes is also referred to as a cell. In simple words the component carrier (CC) means an individual carrier in a multi-carrier system. The term carrier aggregation (CA) is also called (e.g. interchangeably called) “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signalling and data in the uplink and downlink directions. One of the CCs is the primary carrier or anchor carrier and the remaining ones are called secondary or supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signalling. The primary CC exists in both uplink and downlink direction CA. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The CCs belonging to the CA may belong to the same frequency band (a.k.a. intra-band CA) or to different frequency band (inter-band CA) or any combination thereof (e.g. 2 CCs in band A and 1 CC in band B). The inter-band CA comprising of carriers distributed over two bands is also called as dual-band-dual-carrier-HSDPA (DB-DC-HSDPA) in HSPA. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain (a.k.a. intra-band non-adjacent CA). A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible.
In HSPA release 10, up to 4 DL carriers can be aggregated, a.k.a. 4C-HSDPA where the DL carriers or DL cells may belong to the same frequency band or be split over two different frequency bands, e.g. 3 adjacent DL carriers in band I (2.1 GHz) and 1 DL carrier in band VIII (900 MHz). In HSPA Release 11, up to 8 DL carriers may be aggregated and may be called 8C-HSDPA. The DL carriers may be distributed over 2 or more bands. In the present version of the HSPA and LTE specifications (i.e. release 10), all the carriers that belong to one frequency band have to be adjacent when configured by higher layers (e.g. RRC). However, operation on non-adjacent carriers within the same band can result from the carrier activation/deactivation, which is performed by the lower layers (e.g. MAC). In future releases (e.g. release 11) the non-adjacent carriers within the same band may be configurable.
In LTE, intra-band CA, in principle up to 5 DL carriers each of 20 MHz, may be aggregated by the UE. At least, the UE requirements exist for 2 DL carriers, i.e. up to 40 MHz. In LTE inter-band CA, 2 DL carriers belonging to two different bands can be aggregated by the UE.
It is also contemplated to use carrier aggregation of carriers of different RATs, i.e. a multi-RAT multi-carrier concept. Using carrier aggregation between carriers of different technologies is also referred to as “multi-RAT carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity the carrier aggregation within the same technology as described above may be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
It may be a problem with using gaps in measurements on a first carrier for measuring on a second carrier in that information from the first carrier may be lost or the performance of the measurement may be reduced. It would thus be advantageous to be able to improve the use of the measurement resources.